MDL-55720 course: fix potential undefined variable and cleaning

This commit replace $e usage by $returnfromfunc.
Indeed $returnfromfunc always exists in the scope and take $e value in case of moodle_exception.

This commit also remove useless $modcontext affectation.

course/modlib.php

@@ -125,12 +125,11 @@ function add_moduleinfo($moduleinfo, $course, $mform = null) {
     if (!$returnfromfunc or !is_number($returnfromfunc)) {
         // Undo everything we can. This is not necessary for databases which
         // support transactions, but improves consistency for other databases.
-        $modcontext = context_module::instance($moduleinfo->coursemodule);
         context_helper::delete_instance(CONTEXT_MODULE, $moduleinfo->coursemodule);
         $DB->delete_records('course_modules', array('id'=>$moduleinfo->coursemodule));
 
-        if ($e instanceof moodle_exception) {
-            throw $e;
+        if ($returnfromfunc instanceof moodle_exception) {
+            throw $returnfromfunc;
         } else if (!is_number($returnfromfunc)) {
             print_error('invalidfunction', '', course_get_url($course, $moduleinfo->section));
         } else {